Interpretive Summary: Water and solute transport processes at the field scale can be very heterogeneous because of natural spatial variability in soil texture and structure associated with soil type, and because of such external factors as tillage practices and wheel trafficking. The purpose of this study was to understand how the presence of different layers, compartments or compacted zones resulting from plowing, surface tillage and trafficking, affect water and solute transport in an agricultural soil profile. The study involved a detailed field experiment (discussed in a previous companion paper), as well as the use of a numerical model for optimal analysis of the data (described in this paper). The experiment was carried out in a 4m by 2m field plot that was uniformly sprinkle-irrigated with water and a tracer (bromide). Independently measured hydraulic properties (water retention and permeabilities) were used to simulate the data with the HYDRUS-2D numerical flow and transport model. The model reproduced observed flow and transport processes only after adjustments were made to the soil hydraulic functions of the various layers and compacted soil clods. Adjustments were needed to account for increased flow and transport into and through the soil between the compacted zones below the wheel tracks, and to predict double concentration peaks caused by the umbrella (or shadow) effects of compacted soil clods. Detailed studies like this one are helpful to explaining the very heterogeneous nature of field-scale leaching patterns. For example, our modeling results show that irrigular leaching patterms in field soils are sometimes not due to preferential flow through soil macropores (e.g., worm holes or drying cracks), but caused by focused flow around compacted clods that have a lower hydraulic conductivity than other parts of the tilled layer. Such compacted clods are created by compaction under the wheels of farming machinery and subsequent fragmentation and displacement by tillage, especially during moldboard plowing. The compacted clods can significantly affect the water and solute pathways. Water is diverted by the less permeable compacted parts to more permeable soil areas, with soil immediately below the clods being protected from infiltrating water and solutes (referred to as umbrella and shadow effects). Lateral diffusion of water and solute below the compacted clods can then produce a secondary concentration peak. Our results highlight the importance of quantifying the extent and scale of structural heterogeneities that may exist in a soil, with concomitant effects on undesired leaching of agricultural chemicals to underlying groundwater resources. Results are important for better understanding the effects of alternative cropping and tillage systems on water and solute transport processes in the field.

Technical Abstract:
A field experiment was performed to study the effects of soil structure heterogeneity generated by farming practices (i.e., compaction by wheel traffic, plowing, surface tillage) on plot-scale water flow and solute transport. The experiment involved a 4m by 2 m field plot that was uniformly sprinkle irrigated with water and bromide for about 6 h. Independently measured soil hydraulic functions were used to simulate the experiment with a numerical flow and transport model (HYDRUS-2D) using a fully deterministic approach for describing soil heterogeneity. The numerical model reproduced observed flow and transport processes only after adjustments were made to the soil hydraulic functions. Adjustments were needed to account for increased flow and transport into and through the soil between the compacted zones below the wheel tracks, and to predict double concentration peaks caused by the umbrella (or shadow) effects of compacted soil clods. Global optimization of the soil hydraulic parameters produced a satisfactory description of the very heterogeneous flow patterns, with the resulting hydraulic parameters showing only limited correlation among each other. We demonstrate that double-peak concentration profiles can result from the presence of tillage-induced soil heterogeneities.